That's an interesting question. Obviously human beings are more than capable of splitting up into tribes of haters, even in cases such as social media where one can, at no real cost, participate in them all.

But, while generally there's less "holy war" drama of this type on HN than you might find on a generic tech news site, on the topic of Google Plus succeeding there seems to be more.

My personal (and not particularly charitable) assumption is that in addition to the people who'd normally relish a dispute of this type, there's also a subset of people on HN who feel on some level that their experience as early/heavy Facebook adopters will give them some sort of "leg up" in understanding or creating the future, and if something competes with Facebook, it threatens their "insight".

This particular theory aside, it does seem mysterious to me.